I asked google and got this back...
No, you should not lose HD Voice calling. Lebara’s new "NextGen" core continues to support VoLTE (4G Calling) and Wi-Fi Calling, which are the technologies that enable HD-quality audio.
In fact, the transition to their own core is designed to give Lebara more control over these features, though you might notice a few changes:
* Initial Setup: If you switch to an eSIM or a new physical SIM on the "NextGen" platform, it can sometimes take up to 7 days for VoLTE/HD Calling to fully activate on your account.
* Carrier Settings: On iPhones, the network name might change from "Vodafone" to "Lebara" (or even show a generic "Carrier" tag temporarily) in your settings.
* Voicemail: You will move to Lebara’s own voicemail system rather than the one managed by Vodafone.